An expectant mother's diet during pregnancy can alter her baby's DNA in the womb, increasing its risk of obesity, heart disease and diabetes in later life, an international study has found.

Researchers said the study provided the first scientific evidence linking pregnant women's diet to childhood obesity, with major implications for public health.

"This a major breakthrough because for the first time it gives us the potential to work out the optimal diet a mother should eat," Professor Peter Gluckman from Auckland University's Liggins Institute told AFP.

"That's likely to vary slightly from mother to mother, but it could be a major tool in addressing the obesity epidemic."

The study, conducted by scientists in Britain, New Zealand and Singapore, showed that what a mother ate during pregnancy could change the function of her child's DNA through a process called epigenetic change.

Children with a high degree of epigenetic change were more likely to develop a metabolism that "lays down more fat" and become obese, researchers found.

Such children were around three kilograms (6.6 pounds) heavier than their peers by the time they were aged six to nine, Gluckman said.

"That's a hell of a lot of extra weight at that age," he said, adding that the extra fat was likely to be carried into adulthood, raising the chances of developing diabetes and heart disease.

The researchers used umbilical cord tissue to measure the rate of epigenetic change in 300 babies, then examined whether it was linked to the children's weight when they were aged six to nine.

"The correlation was very strong, we didn't believe it at first, so we replicated it again and again," Gluckman said.

The study found the effect was not linked to either the mother or the baby's weight at birth, meaning a slim woman could deliver a small baby which still went on to became obese because of changes triggered by diet in the womb.

Gluckman said the rate of epigenetic change was possibly linked to a low carbohydrate diet in the first three months of pregnancy but it was too early to draw a definitive conclusion and further studies were needed.

He said one theory was that an embryo fed a diet containing few carbohydrates -- which provide the body with energy -- assumed it would be born into a carbohydrate-poor environment and altered its metabolism accordingly.

This meant it stored more fat, which could be used as fuel when food was scarce.

Gluckman said the study, which will be published in the journal Diabetes next week, confirmed long-held suspicions that poor prenatal nutrition could have a major impact on adult heath.

This meant health officials battling soaring obesity rates should look at policies designed to improve the health of expectant mothers, rather than simply focusing on trying to help overweight adults, he said.

"It provides the most compelling argument yet to give greater weight to improving maternal and infant health as a means of reducing the burden of chronic disease."